fringedgentian - 8:50 pm on Jun 6, 2011 (gmt 0)
Yes the data is already on the page, but marking it up makes it that much easier for search engines to extract that data and display it on their page, surrounded by their advertising instead of the advertising for the company who paid for that data to be gathered.
I can definitely see the dilemma from webmasters with proprietary data. That is their bread and butter, and if Google crawls it and starts displaying it on the search results page instead of making users visit your site and view your advertising, buy your products, or however you monetize the data, there goes the entire business model. My opinion is that if Google did that, the company would shut down resulting in everyone's loss: Google, the company, and the user, because now nobody is collecting this data. So I think Google will keep displaying structured data, but in exchange will have to provide enough traffic to keep the data coming (profitable). I think they will track if their displaying of rich snippets causes a marked decrease in click-thrus, and if so they will have to stop displaying that kind of data because that is bad for their own business. They need the content and data producers to stay around.
One last thing I wanted to add is that our website has reviews, and even though we did not add any microdata labeling them as reviews, Google knew what they were and in some cases displayed them in Google Places. So NOT adding the tags does not mean Google won't identify your structured elements and display them regardless. I think Google assumes if you are asking to be crawled and displayed in their search results that you don't care how or where they choose to do it.